Method of preparing molding compositions with fiber reinforcement and products obtained therefrom

ABSTRACT

A method of preparing fiber reinforced acrylic thickened compositions which can be molded under low pressures and temperatures to provide thermoset articles, wherein liquid reactive components are slowly absorbed in high molecular weight solid acrylic resin in the form of large particles. The acrylic resin functions as a thickener which delays the viscosity build allowing fiber reinforcement to be incorporated before molding. The molding composition is well suited for use in dentistry and other fields where small amounts or molding composition are used occasionally.

This is a continuation-in-part application of Ser. No. 09/070,856, filedMay 5, 1998 and now abandoned, which is a continuation-in-part of Ser.No. 08/621,723, filed on Mar. 28, 1996, now issued as U.S. Pat. No.5,747,553, which is a continuation-in-part of Ser. No. 08/429,139 filedon Apr. 26, 1995, now abandoned.

FIELD OF THE INVENTION

This invention relates to the preparation of thermosetting moldingcompositions with an amorphous/non-crystalline acrylic resin thickener,the nature of which permits fiber reinforcement and other additives tobe easily incorporated therein. These thermosetting compositions can bedispensed as premixed doughs (bulk molding compounds) or sheets (sheetmolding compounds) and can be molded using low pressure moldingtechniques and conditions (temperature/pressure) to form articlesranging from automotive parts to dental appliances to bathroom showerstalls.

A. Acrylic Resins

Acrylic resins include polymer and copolymer formulations whose majormonomeric components belong to two families of esters: acrylates andmethacrylates. Acrylics are well known and are commercially availablefrom ICI and others. Trade names such as Elvacite®, Lucite®, Plexiglas®,PERSPEX™ acrylic resins denote these polymer resins.Methyl-methacrylate, ethylacrylate and acrylic acid are common acrylicmonomers. Acrylic monomers can polymerize by a free-radical, additionreaction. Two commercially used initiators for free-radicalpolymerization of acrylic are the peroxide initiator, benzoyl peroxide(BPO) and the aliphatic azonitrile initiator, azobisisobutyronitrile(AIBN). Both BPO and AIBN will decompose into free-radical activators atambient temperature.

Peroxide initiated polymerizations tend to be more vigorous. Peroxidesproduce a higher polymerization exotherm which induces polymer chaindecomposition with subsequent crosslinking between chain fragments.Crosslinking produces three dimensional network polymers that are saidto be thermoset. AIBN and other azonitrile initiators produce lowerpolymerization exotherms, little polymer chain decomposition, minimalcrosslinking and thermoplastic acrylic resins. In addition, azoinitiators irreversibly decompose when heated. Therefore, residual azoinitiator, AIBN, can be eliminated from acrylic resin by the deliberateheating of the resin.

That acrylic monomers can be polymerized in liquid suspensions using azoinitiators such as AIBN is well known. Such azo polymerized vinylpolymers have minimal molecular crosslinking, are thermoplastic and aremore or less soluble in various vinyl monomers. Since thesethermoplastic vinyl polymers are easily molded using injection moldingoperations, they are available from commercial manufacturers. But linearthermoplastic vinyl polymers easily absorb surrounding liquids,including water, with resultant swelling and sometimes dissolution. Sowhere absorbed liquids or thermoplasticity may be a problem,crosslinked, thermoset, acrylic polymers are preferable. Most dentalappliances are molded of thermoset resin. Thermosetting acrylic resinsare the plastic of choice in many industries since the crosslinkedpolymers resist penetration by many liquids and resist distortion byheat or mechanical stress.

B. Acrylic Thickened Molding Compositions/Doughs

Both suspension and emulsion polymerization processes are used toproduce commercial acrylic resins. Suspension polymerization generallyproduces larger beads of resin while emulsion polymerization generates avery, fine powder. Acrylic beads of linear polymers with their highvolume and small surface area are well suited for use in thermoplasticmolding processes and equipment, but acrylic powders are typically usedin forming acrylic thermosetting molding doughs (bulk molding compoundsand sheet molding compounds). Those used in dentistry typically compriseacrylic resin powder, liquid acrylic monomer and an initiator forfree-radical polymerization. Thermosetting molding doughs used forindustrial applications typically contain a thickener such as an alkalimetal oxide. Three types of thermoset dough formulations are commonlyused: those incorporating heat activated initiator; those withchemically activated initiator; and those with radiation (light)activated initiator. A mixture of initiators is used in, "dual cure",resins. Dual cures are typically first light initiated, then heat cured.

Industrial molding doughs, also called premix, are prepared and sold asbulk molding compound (BMC) and sheet molding compound (SMC). Industrialmass producers mix perishable doughs at the job site for high volumeproduction runs, or purchase another premix for molding within a fewweeks of its preparation. The alkaline earth thickeners, peroxidecatalysts and promoters in the dough cause gelation that limits shelflife. Commonly used industrial, alkaline earth thickeners, CaO or MgO,polymerize and build dough viscosity indefinitely.

Thermoset plastic dental fillings, dental crowns and dental prosthesisare most often made by low pressure compression molding of the acrylicdough. To maintain viscosity, the thermosetting acrylic thickened doughsused in dentistry or other small or occasional applications is typicallyprepared on site due to the short shelf life characteristic.

The thermosetting acrylic molding doughs contain powdered acrylic resinrather than large beads because 1) their rapid solubility in acrylicmonomer quickly dissolves the resin into the monomer; and 2) the solubleresin powder contains enough residual initiator to trigger thepolymerization of a lightly inhibited monomer liquid. In dentalpractice, these finely powdered polymers are also of low molecularweight of about 60,000 so as to be readily soluble in the acrylicmonomer liquid.

C. Fiber Reinforcement

Although the acrylic doughs provide useful molded articles, the physicalproperties are not ideal for all applications and have been manipulatedby blending methyl methacrylate with other resins, forming copolymerswith the methyl methacrylate monomer and/or increasing the degree ofcrosslinking between polymer chains. Adding fiber reinforcement isdesirable for some applications.

Long fibrous fillers, such as glass, carbon, aramid, etc., are known togreatly enhance strength, stiffness and toughness of plastic materials.Long fibers being defined as lengths equal to or exceeding the criticalaspect ratio of the fiber matrix combination. Plastics reinforced withsuch long fibrous inclusions, i.e., composites, exhibit physical andchemical properties that are a composite of the properties of thefibrous fillers and plastic matrix. Typically, the included fiber hastensile strength much higher than the resin matrix, is insoluble in theresin matrix and is chemically, or physically bonded to the resin matrixin such a way as to deflect a crack propagating through the resin matrixalong the length of the fiber-matrix interface. Fibers turn the crack,absorb the energy of fracture, reduce the incidence ofthrough-and-through-fracture, and give composites their characteristicproperties of high strength, high stiffness, toughness and light weight.The properties of some conventional polymeric materials and compositesare disclosed in CRC Practical Handbook of Materials Science, Ed.Charles T. Lynch, 1994, pp. 547-548 (vinyls, ASA resins), 327-328 (glassfiber, organic fiber) and 342 (organic matrix composites). While the useof long fibrous fillers can provide advantageous physical properties,fiber is difficult to incorporate into a resin matrix, particularlywhere the matrix resin is highly viscous.

The thermosetting acrylic thickened doughs of powdered, low molecularweight acrylic resin and lightly inhibited acrylic monomer liquid arehighly viscous. The mixture quickly passes from a wet slurry, to aviscous paste and then to a moldable dough as the resin particles firstabsorb and then dissolve in the monomer. Unfortunately, this otherwiseconvenient, rapid transition from slurry to paste to dough produces avery abrupt rise in viscosity. In addition, residual BPO initiator inthe acrylic resin powder, intended to thermoset the mix, beginsspontaneous decomposition, initiates polymerization and can reduce theshelf life of the fresh dough to only a few hours at 80° F.

Early efforts to bring a thermoset, fiber reinforced, organic polymercomposite to the dental market have foundered on the two problems of 1)prematurely high viscosity; and 2) premature gelation (polymerization).Viscosity is intentionally built-up rapidly in dental acrylic as finelypowdered, low molecular weight resins are dissolved in monomer to make amoldable dough. Premature gelation is a consequence of the demand forlow curing temperature. Adding reinforcing fibers to this increasinglyviscous mix quickly becomes impractical. Adding long fibers to anyliquid typically causes the liquid to become intractably thick, even atlevels of only 2 wt. %. As disclosed by J. E. Gordon in The New Scienceof Strong Materials, 2nd Ed., p. 177, Princeton Univ. Press, "Beyond twopercent, therefore, it is impossible to add fibers to a matrix and itbecomes necessary to add the matrix to the fibers." Prolonged mixingwith significant energy input and subsequent heat build up is requiredto incorporate fibers into liquid monomer and wet the fiberreinforcement.

Dental researchers have long wrestled with the problem of incorporatingreinforcing fibers into thermoset, molding doughs. For example,Ladizesky, Chow and Cheng, using a cloth, impregnated with acrylicsyrup, disclose, "The added technical procedure to construct the (fiber)reinforced dentures required an additional two hours (20%) of thestandard laboratory time." Denture Base Reinforcement Using WovenPolyethylene Fiber, International Journal of Prosthodontics, Vol. 7, No.4, p. 307-314 (1994). Targis® by Ivoclar is an example of a commercialpre-preg used in dentistry.

In the 1960's, Bowen, U.S. Pat. No. 3,066,112, incorporated particulateglass fillers into acrylic and vinyl ester resins used as dentalfillings. Since then, small particulate fillers have been used to reducethe shrinkage of polymerization, increase hardness and improve abrasionresistance of these dental materials. However, these particle filledmaterials do not behave as fiber reinforced composites. Unless thecritical aspect ratio, length/diameter, of a reinforcing fiber embeddedin a resin matrix is equaled or exceeded, the composite material failsat low stress levels. Early dental composites did not containparticulate fillers with aspect ratios exceeding 4/1. In dentalpractice, particulate fillers are nearly spherical to enhance flow andmixing. Consequently, there is very little resistance to crackpropagation in these composites. The result is very little enhancementin strength, stiffness and toughness of the dental composite material.Short particle fillers can actually make the dental materials brittle.

Fiber reinforcement has been incorporated in thermosetting moldingdoughs on an industrial scale and fiber reinforced industrial moldingdoughs are available as bulk molding compound (BMC) or sheet moldingcompound (SMC). However, these compounds have extremely high viscositiesof 20-30 million centipoise. These high viscosities are manageable onthe industrial scale where large hydraulic or electric presses can beused to generate the high molding pressures and temperatures necessaryto mold these compounds. It is desirable to reduce these pressure andtemperature requirements to enable molding of fiber reinforced resinsunder low pressure molding conditions.

Short shelf life, high molding pressure and temperature requirementshave prevented the commercial production and distribution of fiberreinforced, polyester/acrylic, vinyl ester and acrylic, molding doughsto the very small user doing an occasional or opportunistic molding.Perishable, industrial BMC and SMC has, until now, been unsuitable forthe small batches of premix used on the occasional, very small job ofthe dentist, the auto body mechanic, the boat repair person or the like.The small of occasional user, like the dentist, requires a doughmoldable with manual pressure, curable at hot water temperatures andwith a long shelf life at ambient temperature for their opportunistictype of work.

The use of additives to reduce the viscosity of the molding dough forlow pressure molding has had limited success. See: Proceedings,Composites Institute 51st Annual Conference and Expo 96. A low meltingcrystalline polyester resin available under the tradename CRYSTIG™polyester resin, imparts low pressure qualities to the moldingdough/composition when melted at a temperature of over 100° C. andsubsequently cures. This requires the reinforcement be incorporated inthe melt just before use. It is desirable to provide a fiber reinforcedthermosetting molding dough which is not so limited.

Three factors prevent the easy, on site mixing of resin powders andreinforcing fibers, with curable liquid monomers.

1) Resin particles and fibers tend to separate into layers and clumps,called agglomeration, and require periodic stirring to prevent thisseparation.

2) Prolonged mixing is required to incorporate particles and fibers intoliquid monomer and thoroughly wet the particulate fillers and fiberreinforcement. If the resin particles dissolve immediately, viscositybuild up prevents farther mixing.

3) Dry ingredients must be very dry. Water contaminated powder and fiberwill contaminate and weaken the composite. Surface moisture, adsorbed atambient temperature, must be removed from particles and fibers so themonomer can wet and bond to these ingredients. Interfacial bondingbetween solid fillers and curable liquids must occur duringpolymerization if physical properties are to be enhanced rather thandegraded in the composite.

Drying and mixing require time and special equipment not available tothe opportunistic molder working at the occasional job.

SUMMARY OF THE INVENTION

It is an object of this invention to provide fiber reinforcedthermosetting bulk molding compounds (BMC) and sheet molding compounds(SMC) and precursors thereto with extended shelf stability.

It is another object of this invention to provide a thickener forthermosetting molding compositions (BMC, SMC) which delays viscositybuild-up to provide an opportunity to incorporate long fiberreinforcement and employ low pressure molding techniques withoutheating.

It is another object of this invention to provide a thickener forthermosetting molding compositions which thickens by a physicalmechanism and not a chemical mechanism, and participates in the cure.

It is another object of this invention to provide thermosetting moldingcompositions, precursors thereto and methods for their preparation,which allow long fiber reinforcement to be easily incorporated therein.

It is another object of this invention to provide fiber reinforcedthermosetting bulk molding compounds (BMC) and sheet molding compounds(SMC) and precursors thereto which can be molded under the pressures andtemperatures of low pressure molding equipment.

It is an another object of the present invention to provide athermosetting acrylic thickened composition with fiber reinforcementsuitable for industrial applications or dental appliances with fiberreinforcement that enhances the physical properties of the moldedproduct.

It is another object of the present invention to provide a thermosettingmolding composition suitable for dental appliances which formscomposites of suitable strength to replace the metal frameworks andsuperstructure employed to reinforce and support dental crowns and fixedand removable dental bridge work.

It is a another object of the present invention to provide athermosetting molding composition with fiber reinforcement which formsdental appliances of suitable strength to replace those produced bylost-wax casting and ceramic build-ups, with reduced fabrication time.

It is another object of the present invention to provide a thermosettingpremixed acrylic-based molding composition with fiber reinforcementsuitable for industrial applications or dental appliances to reduceexposure of the operator (and patient) to hazardous vapors.

It is another object of the present invention to provide a thermosettingmolding composition which has an extended shelf life so as to reducewaste.

It is an additional object of the present invention is to provide athermosetting molding composition which experiences less shrinkage uponcure, requiring fewer adjustments (secondary finishing), to complete thepart.

It is a further object of the present invention to provide athermosetting molding composition with long fiber reinforcement which iscompatible with existing techniques, equipment and procedures forproducing dental appliances.

These and other objects are achieved through the methods andcompositions of this invention which comprises molding compositions andprecursors thereto. This includes bulk molding compositions and sheetmolding compositions. BMC and SMC are composed basically of fourprinciple ingredients: thermosetting resins (polymerizable resinsolution), fibers, optionally fillers and additives. With this overallcombination in place, it is feasible to use various types of specificingredients to meet the required properties of the final product, andthat makes BMC and SMC very versatile reinforced composites with analmost indefinite number of possible formulations. A general formula oftypical sheet molding and bulk molding compounds, employing a variety offree radical initiators and other ingredients, which can be used in themethod of thickening polymerizable resin solutions into moldablecompositions provided by this invention follows.

Suitable polymerizable resin solutions are made by dissolving a curableresin polymer, in a curable monomer, oligomer or polymer solvent, as ameans of making a curable molding composition, e.g.:

a) acrylic resins dissolved in a polymerizable monomer solvent, thepolymerizable monomer solvent dissolving both the acrylic resin and "thethickener", examples of the polymerizable monomer solvent includestyrene monomer, acrylic monomer, EGDMA (ethylene glycoldimethacrylate), TEGDMA (triethylene glycol dimethacrylate), UEDMA(urethane dimethacrylate) or similar acrylates;

b) polyester resins dissolved in a polymerizable monomer solvent, saidpolymerizable monomer solvent dissolves both the polyester resin and"the thickener", e.g., styrene monomer, acrylic monomer, EGDMA, TEGDMA,UEDMA or similar acrylates;

c) styrenic resins dissolved in a polymerizable monomer solvent, saidpolymerizable solvent dissolving both the styrenic resin and "thethickener", e.g., styrene monomer, acrylic monomer, EGDMA, TEGDMA, UEDMAor similar acrylates; and

d) vinyl ester resins dissolved in polymerizable monomer solvent, saidpolymerizable solvent dissolving both the vinyl ester resin and "thethickener," e.g., styrene monomer, acrylic monomer, EGDMA, TEGDMA, UEDMAor similar acrylates.

The addition of fiber(s) provides a means for strengthening orstiffening the polymerized resin solution. The fiber(s) often used are:

1) inorganic crystals or polymers, e.g., fibrous glass, quartz fibers,silica fibers, fibrous ceramics, e.g., alumina-silica (refractoryceramic fibers); boron fibers, silicon carbide whiskers or monofilament,metal oxide fibers, including alumina-boria-silica,alumina-chromia-silica, zirconia-silica, and others;

2) organic polymer fibers, e.g., fibrous carbon, fibrous graphite,acetates, acrylics (including acrylonitriles), aliphatic polyamides(e.g., nylons), aromatic polyamides, olefins (e.g., polypropylenes,polyesters, UHMW polyethylenes), polyurethanes (e.g., spandex,alpha-cellulose, cellulose, regenerated cellulose (e.g., rayon), jutes,sisals, vinyl chlorides (e.g., nylon), vinyl chlorides (e.g., vinyon),vinyldienes (e.g., saran), flax and thermoplastic fibers;

3) metal fibers, e.g., aluminum, boron, bronze, chromium, nickel,stainless steel, titanium or their alloys; and

4) "Whiskers", single, inorganic crystals.

Suitable nonfibrous filler(s) are inert, particulate additives beingessentially a means of reducing the cost of the final product, whichoften reduce some physical properties of the polymerized, resin-fibercomposite and are optional. Examples include calcium carbonates ofvarious forms and origins, silica of various forms and origins,silicates, silicon dioxides of various forms and origins, clays ofvarious forms and origins, feldspar, kaolin, flax, zirconia, calciumsulfates, micas, talcs, wood in various forms, glass (milled, platelets,spheres, micro-balloons), plastics (milled, platelets, spheres,micro-balloons), recycled polymer composite particles, metals in variousforms, metallic oxides or hydroxides (except those that alter shelf lifeor viscosity), metal hydrides or metal hydrates, carbon particles orgranules, alumina, tabular, aluminum powder, aramid, bronze, carbonblack, carbon fiber, cellulose, alpha cellulose, coal (powdered),cotton, fibrous glass, graphite, jute, molybdenum, disulfide, nylon,orlon, rayon, silica, amorphous, sisal fibers, fluorocarbons and woodflour.

Suitable additives include initiators and thickener(s). Initiators arethe means of generating the free radicals that begin and sustainpolymerization. Said initiator-monomer combination, otherwise stable forat least a week at ambient temperature, is activated by means ofelevating temperature, or by exposing to microwave, infrared, visible,ultra-violet or shorter radiations thus generating free radicals.Specific initiators are described below.

Acrylic beads are used as the means of thickening the polymerizableresin solution into a moldable dough. Said solid acrylic beads being ofan essentially linear, acrylic polymer act by slowly absorbing thepolymerizable monomer resin solvent. The solid, acrylic bead thickenersare essentially free of initiators as a means to extend shelf life.These acrylic bead thickeners are of a molecular weight, chemicalcomposition and large bead diameter as a means to be both slowlydissolving in and highly absorbing of the polymerizable monomer,oligomer or polymer solvent.

The following are functional additives and a means of impartingdesirable properties to the molding composition or to the curedcomposite. These include, but are not limited to, anti-blocking agents,anti-caking agents, anti-foaming agents, antioxidants, anti-slip agents,anti-static agents, blowing agents, coupling agents, compatibilizers,dispersing aids, flatting agents, inhibitors, catalysts,accelerators/promoters, heat stabilizers, light stabilizers, wettingagents, plasticizers, extenders, thixotropics, flame, fire and smokeretarders, internal mold releases, lubricants, impact modifiers,tougheners, coloring/dyes/pigments, odorants and deodorants, low profileor low shrink additives, low pressure additives, clarifying agents,opacifiers, thickeners, viscosity control agents, permeabilitymodifiers, solvents, waxes and thermoplastics.

The molding compositions preferably comprise:

a) a liquid monomer, oligomer, polymer or combination thereof,containing vinyl unsaturation, which polymerizes in the presence of anactivated free-radical polymerization initiator;

b) at least 35 wt. %, based on the total weight of liquid monomer,oligomer, polymer or combination thereof containing vinyl unsaturationin the composition, of a solid acrylic resin which is soluble in saidliquid monomer, liquid oligomer, liquid polymer or combination thereofcontaining vinyl unsaturation, having an average particle size in therange of 0.005 mm to 0.5 mm with at least a portion of said liquidmonomer, oligomer, polymer or combination thereof containing vinylunsaturation, absorbed therein, said solid acrylic resin is free ofactive free-radical polymerization initiators and reacts with the liquidmonomer, oligomers, polymers and combinations thereof, absorbed thereinin the presence of an activated free-radical polymerization initiator;

c) at least 10 wt. %, based on the total weight of the composition, oflong fiber reinforcement having an aspect ratio (L/D) greater than 5:1and an average length of at least 0.25 mm which is insoluble in thesolid acrylic resin; and

d) a free-radical polymerization initiator, the activity of which can berestrained under ambient conditions or is inactive at ambienttemperature so as to provide a shelf life of at least one month atambient temperature. The molding compositions are free of alkali earthmetal oxide fillers/thickeners.

The precursors to a molding composition comprise:

a) a thermosetting resin in solution of a curable liquid monomer, liquidoligomer, liquid polymer or combination thereof containing vinylunsaturation, which polymerizes in the presence of an activatedfree-radical polymerization initiator;

b) about 35 wt. %, based on the total weight percent of the liquidmonomer, oligomer, polymer or combination thereof in the composition, ofa solid acrylic resin which is soluble in said liquid monomer, oligomer,polymer or combination thereof containing vinyl unsaturation, which hasan average particle size in the range of 0.005 mm to 0.5 mm, with atleast a portion of said liquid monomer, oligomer, polymer or combinationthereof containing vinyl unsaturation absorbed therein and which is freeof free-radical polymerization initiators; and

c) at least 10 wt. %, based on the total weight of the composition, oflong fiber reinforcement having an aspect ratio (L/D) greater than 5:1and an average length of at least 0.25 mm.

The precursor compositions are shelf stable for at least one month andare free of alkali earth metal oxide fillers/thickeners and activefree-radical initiators.

The methods comprise:

a) mixing a solid acrylic resin, which is free of active free-radicalpolymerization initiators, with

(i) one or more liquid monomers, liquid oligomers, liquid polymers or acombination thereof with vinyl unsaturation which polymerizes in thepresence of an activated free-radical polymerization initiator, whereinsaid solid acrylic resin absorbs the liquid monomers, liquid oligomers,liquid polymers or combination thereof containing vinyl unsaturation,and reacts with said liquid monomers, liquid oligomers, liquid polymersand combinations thereof, absorbed therein, in the presence of anactivated free-radical polymerization initiator; and

(ii) long fiber reinforcement having an aspect ratio L/D greater than5:1, which is insoluble in said solid acrylic resin; and

b) aging the mixture of solid acrylic resin, liquid monomer, liquidoligomer, liquid polymer or combination thereof containing vinylunsaturation, and long fiber reinforcement for at least 24 hours toallow absorption of the liquid monomer, oligomer, polymer or combinationthereof, by the solid acrylic resin.

This invention involves the preparation of novel compositions of stable,thermosetting, acrylic, styrenic, vinyl ester or polyester thickeneddoughs. Preferred embodiments of these compositions can be compounded tobe compression molded at low pressure such as that from the manual screwor small hydraulic press familiar to a dental laboratory or low pressuremolding equivalents. The preferred compositions can be cured at lowtemperatures using a hot water bath or UV or visible light sourcefamiliar to a dental laboratory. These compositions include a highlyabsorbent, solid acrylic resin which functions as a thickener and allowsthe economic, bulk manufacture and packaging of fiber reinforcedpolyester, vinyl ester, styrenic or acrylic thickened compositions. Thissame highly absorbent, solid acrylic resin allows a later distributionof the small quantities of these molding compositions to users remote intime and place from their site of manufacture.

The fiber reinforced acrylic, vinyl ester/acrylic and polyester/acrylichave a combination of sufficiently:

1) long shelf life;

2) low molding pressure; and

3) low curing temperature to be practical for the dentist or, the lowproduction volume, custom molder, or the occasional molder at theopportunistic job.

The thickening agent is preferably a solid soluble, highly absorbent,high molecular weight and unbranched thermoplastic acrylic resin. Thisthickener defeats the problems of:

1) Rapid viscosity build up during mixing of ingredients which preventsthe thorough mixing and wetting of fillers and fibers by the liquid; and

2) Premature dough gelation.

The slowly soluble solid acrylic resin allows prolonged mixing of fiber,filler and liquid ingredients for periods as long as one hour. The slowdissolution of the solid acrylic resin delays viscosity build up for alength of time required to thoroughly mix and wet reinforcing fibers inthe premix slurry. This unique acrylic resin thickening agent can actentirely without alkaline earth additives. During a period ofmaturation, typically 1-4 days, the solid acrylic resin absorbs themonomer solvent and dissolves to form a curable dough. This period ofmaturation allows the thickener to convert the wet slurry first into apaste and then into a dough. This dough can remain moldable at a lowpressure for many months and as long as two years when totally devoid ofunstable initiators of polymerization such as benzoyl peroxide or azoinitiators. Since the solid acrylic resin thickener is devoid of activeinitiators, stable initiators such as t-butyl peroxybenzoate andinhibitors can be incorporated into the molding compositions to avoidpremature gelation of the dough. This facilitates a long shelf life.

The preferred acrylic resin thickener is an unbranched polymethylmethacrylate resin (PMMA) polymerized with an azo initiator in asuspension polymerization to a molecular weight of about 400,000 asdetermined by GPC using a conventional solvent for PMMA resins in about0.1 mm particles/beads. An amount of 0.25 grams of a 400,000 molecularweight polymer dissolved in 50 ml of methylene chloride measured at 20°C. using a No. 50 Cannon-Fenske viscometer has an inherent viscosity of1.25. These resins are commercially available from ICI Chemical underthe tradename Elvacite® 2051.

This preferred thickening agent, a thermoplastic, solid acrylic resin,is created by raising the temperature of the resin above thedecomposition temperature of the azo initiators to eliminate residualinitiator. This can be done in the autoclave immediately followingsuspension polymerization. Or, the resin particles/beads can be baked atup to 100° C. Either method decomposes and eliminates residual azoinitiator. Baking may cause the particles/beads to stick together inaggregates. The baked particle/bead aggregate can be tumbled in a drummixer for 30 minutes to break up clumps of resin beads. Preferably thepolymer is baked at a temperature above the decomposition temperature ofthe azo initiator but below the polymer's glass transition temperatureto avoid resin fusion.

A large particle/bead size minimizes the soluble exposed surface areaand a very large high molecular weight minimizes polymer solubility.Various combinations of bead size and molecular weight make the beadsmore or less soluble during compounding. It's relative insolubilityallows the resin to mix with a liquid monomer, oligomer or polymer,preferably methyl methacrylate, without producing a noticeable immediateincrease in viscosity. The preferred thickening agent slowly absorbs theliquid, swells and dissolves during a one to four day maturation periodin a sealed container at 70° F. The incubator is inverted at least onceevery 24 hours. During this maturation period, the viscosity of the wetslurry increases to a paste and then plateaus at a doughy consistencyhaving more or less tack and viscosity depending on the nature and theratios of liquid to solid ingredients.

With long fiber reinforcement incorporated therein, the moldingcomposition provides thermoset articles, including dental appliances,which are composites with a unique property profile. These compositescan substitute the metal frameworks and superstructure used to supportdental crowns and bridge work. These composites also provide analternative to ceramic build-ups and appliances made by lost waxcasting. In addition to enhancing physical properties, the fiberreinforcement reduces shrinkage in the molded article, requiring feweradjustments and finishing steps.

The relatively long shelf-life of the molding compositions of thepresent invention of at least one week provides adequate time touniformly blend in the fiber reinforcement, even where mixing forextended periods of over one hour is required. Where the shelf-lifeextends beyond one year, premixes can be prepared and waste is reduced.The extended shelf life is determined by the initiator and the additivesutilized. Preferably, benzoyl peroxide catalyst is avoided, unless itsactivity at ambient temperature is suppressed, and the use of alkalimetal oxide fillers to thicken the formulation is avoided.

The term "acrylic resins" as used herein is intended to include acrylicmonomers of the structure: ##STR1## wherein R═H or a hydrocarbon basedradical, and

R¹ =a hydrocarbon based radical;

and oligomers, polymers and copolymers thereof. Included within theacrylic polymers are linear, branched and cross-linked homopolymers.Included within the acrylic copolymers are graft copolymers, randomcopolymers, block copolymers and crosslinked copolymers with two or moreacrylate monomers of formula I or different monomers such as styrene andacrylonitrile (ASA resins) and acrylamide and methacrylamide. Thepreferred acrylic resins are the monomers, polymers and copolymers, bothlinear and cross-linked, of methylmethacrylate and ethylmethacrylate.Suitable comonomers include amino methacrylates available from Creanovasuch as N,N-Dimethylamineothyl Methacrylate, N-DimethylaminopropylMethacrylamid, and 2-ter.-Butylamineothyl Methacrylate.

The hydrocarbon based radicals of R and R¹ include methyl, ethyl,propyl, isopropyl, and n-butyl, sec-butyl, isobutyl, tert-butyl, hexyl,heptyl, 2-heptyl, 2-ethylhexyl, 2-ethylbutyl, dodecyl, hexadecyl,2-ethoxyethyl isobornyl and cyclohexyl. Preferred acrylates have R andR¹ selected from the C₁ -C₄ series. The most preferred acrylic polymeris based on methylmethacrylate. A preferred methylmethacrylate acrylicpolymer is Elvacite® 2051, available from ICI. Other examples includethose provided by Creanova Inc. such as methacrylic acid, EthylMethacrylate, n-Butyl Methacrylate, Iso-Butyl Methacrylate, StearlyMethacrylate, Cyclohexyl Methacrylate, 2-Ethylhexyl Methacrylate,Methacrylic Ester, Lauryl Methacrylate, n-Hexyl Methacrylate, IsobornylMethacrylate, Benzyl Methacrylate, Allyl Methacrylate,3,3,5-Trimethyl-Cyclohexyl Methacrylate, 1,4-Butanediol Dimethacrylate,1,3-Butanediol Dimethacrylate, 1-3,-Butanediol Dimethacrylate,Polyethylene Glycol Dimethacrylate, Triethylene Glycol Dimethacrylate,Diethylene Glycol Dimethacrylate, 1,6-Hexanediol Dimethacrylate,Tetratethylene Glycol Dimethacrylate, TrimethylolpropaneTrimethacrylate, Timethylolpropane Trimethacrylate, 1,1 2-DodecanediolDimethacrylate, Glycerol-1,3-Dimethacrylate, Diurethane Dimethacrylate,2-Hydroxyethyl Acrylate, Hydroxypropyl Acrylate, 2-HydroxyethylMethacrylate, and Glycerolmonomethacrylate. The acrylic resins oftenhave polmerization inhibitors such as methlethyl hydroquinoneincorporated in amounts less than 0.1%.

The term "acrylic resins" as used herein is also intended to includevinyl ester resins such as those derived from Bis-GMA. Bis-GMA isessentially an oligomer of the formula ##STR2## wherein R is ##STR3##which can be obtained by reaction of one molecule of bisphenol-A and 2molecules of glycidylmethacrylate or by reaction of diglycidylether ofbisphenol-A with methacrylic acid. Similar vinyl esters can be preparedusing other polyepoxides and unsaturated monocarboxylic acids. Theseresins are cured at ambient or elevated temperatures by free-radicalpolymerization in a manner analogous to the acrylic resins containingmonomers of formula (I) above.

The term "solid acrylic resin" as used herein is intended to includepolymers and copolymers of the acrylate monomers described above andpolymers produced from Bis-GMA described above.

The compositions of this invention include a liquid monomer, liquidoligomer or liquid polymer with vinyl unsaturation which cures to athermoset polymer in the presence of a free-radical polymerizationinitiator. The liquid monomer oligomer or polymer must also be able tosolubilize the solid acrylic resin so that the liquid monomer, oligomeror polymer will be absorbed by the solid acrylic resin. Suitable liquidmonomers, oligomers and polymers include the liquid acrylic monomersdescribed above and liquid oligomers (diacrylates and dimethacrylates)and polymers obtained therefrom. Suitable liquid oligomers and polymersalso include the liquid Bis-GMA oligomers and polymers described aboveand further include liquid polyester resins, liquid carboxylatedacrylates, liquid carboxylated methacrylates, liquid styrene monomersliquid substituted styrene monomers and oligomers. Examples ofdimethacrylates include ethylene glycol dimethacrylate (EGDMA),triethylene glycol dimethacrylate (TEGDMA) and urethane dimethacrylate(UEDMA).

The compositions of this invention contain at least a portion of solidacrylic resin, preferably about 35 wt. %, most preferably 35-70 wt. % ofthe total weight percent of the liquid monomer, oligomer, polymer orcombination thereof in the composition. The solid acrylic resinfunctions as a highly absorbent thickener which participates in thecure. Suitable solids are sold under the trade name Elvacite® 2051 byICI. This highly absorbent solid acrylic resin is amorphous andthermoplastic and preferably a linear and unbranched homopolymer. Thesolid acrylic resin preferably has 1) a high molecular weight,preferably above 100,000, most preferably about 400,000 as determined byconventional gel permeation chromatography (GPC) methods usingconventional solvents for acrylic resins, such as methylene chloride; 2)a large particle size, preferably about 0.005 mm to 0.1 mm and mostpreferably about 0.1 mm; and 3) essentially no active free-radicalpolymerization initiators. The solid acrylic resin preferably has aninherent viscosity of about 1.25 as tested in a Cannon-Fenske viscometerwith 0.25 grams in 50 ml of methylene chloride at 20° C.

The solid acrylic resin absorbs solvent which in the composition of theinvention is the liquid monomer, oligomer or polymer. The function ofthis solid acrylic resin within the molding composition is to provide adelayed viscosity build, thus permitting the prolonged mixing necessaryto incorporate thoroughly wet high volumes of filler and fiber into themolding composition. Absorption of the liquid monomer, oligomer orpolymer is preferably not substantially complete until at least 2 hoursafter being mixed with the solid acrylic resin. Most preferably,absorption of the liquid monomer, oligomer or polymer by the solidacrylic resin (viscosity build) is substantially complete (about 90%)within 1 to 4 days from forming a mixture thereof.

The delayed absorption of the liquid provides for a low viscosity whichis sufficiently low to enable both molding compounds and sheet moldingcompounds to be molded under the temperatures and pressures of lowpressure equipment.

The amount of acrylic resin (acrylics and vinyl esters) within thecompositions of this invention can vary widely, particularly whenemployed with other compatible resins. The amount of acrylic resin(liquid and solid) preferably ranges from 35 to 95 wt. % of the liquidmonomers, oligomers and/or polymers in the composition, more preferablyfrom about 50-70 wt. % of these liquid components in the composition.Acrylic resins can form 100% of resin component of the compositionexcept where vinyl ester resin is the "acrylic resin".

The compositions of the present invention can include other solid orliquid resins which will participate in the free-radical polymerizationor solid resins which remain inert during polymerization, functioning asorganic fillers or other additive. Essentially any liquid or solid vinylor diene containing monomer, oligomer, polymer or copolymer which willparticipate in free-radical polymerization at ambient temperature can beused. These include polyesters and those derived from the monomersselected from the group consisting of vinyl ethers, acrylonitrile,styrene, propylene, vinyl acetate, vinyl alcohol, vinyl chloride,vinyldiene chloride, butadiene, isobutylene, isoprene, divinylbenzeneand mixtures thereof. An example of an inert resin is polyethylene,which in particulate form can function as an organic filler. However, itis preferable that acrylic resins, i.e., those derived from the monomersof formula 1 and the vinyl ester resins be used exclusively in themolding compositions of this invention. Examples of suitable solidacrylic resins are methacrylate polymers under the trademark ROHAGUM®,ROHAMERE® and ROHADON® available from Creanova.

Embodiments of this invention include thermosetting molding compositionsand precursors thereto. The thermosetting molding compositions of thepresent invention include a free-radical polymerization initiator. Thisinitiator can be any conventional free-radical initiator. The initiatorpreferably has an activity which can be restrained (inhibited/retarded),preferably at ambient conditions and most preferably elevatedtemperatures. Free-radical initiators which initiate polymerization byexposure to either elevated temperatures above ambient temperatureand/or exposure to UV or visible light are well suited for providingmolding compositions with the requisite shelf stability of at least oneweek. Suitable temperature activated initiators include t-butylperoxybenzoate, sold under the trade name Trigonox® by Akzo ChemicalsInc., t-butyl hydro-peroxide and the peroxy ketals, also available fromAkzo Chemicals Inc. and the VAZO catalysts such asVAZO-88™1,1-azobi(cyclohexane carbonitrile) available from DuPont. Othersuitable initiators include ketone peroxides, alkyl peroxides, arylperoxides, peroxy esters, perketals, peroxydicarbonates,alkylhydroperoxides, diacyl peroxides, VAZO compounds, photoinitiatorsand heat labile photoinitiators.

Examples of ketone peroxides include methyl ethyl ketone peroxide,methyl isobutyl ketone peroxide, acetyl acetone peroxide andcyclohexanone peroxide.

Examples of alkyl peroxides and aryl peroxides include dicumyl peroxide,tert.butylcumyl peroxide, di-tert.amyl peroxide,1,3-di(2-tert.butylperoxy isopropyl)benzene, di-tert.butyl peroxide,2,5-dimethyl 2,5-di(tert.butylperoxy)hexane, 2,5-dimethyl2,5-di(tert.butylperoxy)hexane, dibenzoyl peroxide, tert.butyl3-isopropenylcumyl peroxide, 1,4-di(2-tert.butylperoxyisopropyl)benzene, 1,4-di(2-neodecanoyl peroxy isopropyl)benzene,di(1-hydroxycyclohexyl) peroxide, diisobutyryl peroxide, dioctananoylperoxide, didecanoyl peroxide and2,2-Bis(4,4-di(tert.butylperoxy-cyclohexyl)propane.

Examples of peroxyesters include tert-butyl peroxy-2-ethylhexanoate(Trigonox 21), tert-amyl peroxy-2-ethylhexanoate (Trigonox 121),tert-butyl peroxy-3,5,5-trimethylhexanoate (Trigonox 42S),tert-butylperoxy-2-methylbenzoate (Trigonox 97-C75), 2,5-dimethyl2,5-di(benzoylperoxy)hexane (AZTEC),2,5-dimethyl-2,5-di-(2-ethyl-hexanoylperoxy)hexane (Trigonox 141),tert-butyl peroxy-isopropyl carbonate (Trigonox BPIC), tert-butylperoxy-stearyl carbonate, tert.-butyl peroxyacetate, tert.-amylperoxyacetate, tert.-butyl peroxypivalate, tert.-amyl peroxypivalate,tert.-butyl peroxyneodecanoate, tert.-amyl peroxyneodecanoate,tert.-butyl peroxybenzoate, tert.-amyl peroxybenzoate (Trigonox 127),tertiary-butyl peroxy 2-ethylhexyl carbonate, tertiary-amyl peroxy2-ethylhexyl carbonate, cumyl peroxyneodecanoate, cumylperoxyneoheptanoate, tertiary-butyl peroxyneoheptanoate, tertiary-butylperoxyisobutyrate, tertiary-butyl monoperoxy maleate and tert.-butylperoxydiethyl acetate.

Examples of perketals include 1,1-di(t-amylperoxy)cyclohexane (USP-90MD)2,2,-di(tert.butylperoxy)butane, n-butyl4,4-di(tert.butylperoxy)valerate ethyl 3,3-di(tert.butylperoxy)butyrate,3,3,6,6,9,9-hexamethyl 1,2,4,5-tetraoxa cyclononane,1,1-di-(tert-butylperoxy)-3,3,5-trimethylcyclohexane,1,1-di-(tert-butylperoxy)cyclohexane and di-tert.-butyldiperoxyazelate.

Examples of peroxy dicarbonates include di-(sec-butyl)peroxydicarbonate,di-(n-butyl)peroxydicarbonate, di-(2-ethylhexyl)peroxydicarbonatedi-(4-tert-butylcyclohexyl)peroxydicarbonate, dicyclohexylperoxydicarbonate, dimyristyl peroxydicarbonate and dicetylperoxydicarbonate.

Examples of alkylhydroperoxides include cumene hydroperoxide,1,4-di(2-hydroperoxy isopropyl)benzene, tert.amyl hydroperoxidetert.butyl hydroperoxide, 2,4,4-trimethylpentyl-2 hydroperoxide anddiisopropylbenzene monohydroperoxide

Examples of diacyl peroxides include acetyl cyclohexane sulphonylperoxide, di(2,4-dichlorobenzoyl)peroxide, di(3,5,5-trimethylhexanoyl)peroxide dilauroyl peroxide, disuccinic acid peroxide anddi(4-methylbenzoyl)peroxide.

Examples of VAZO compounds include2,2'-azobis(2,4-dimethylpentanenitrile),2,2'-azobis(2,4-dimethylvaleronitrile) (Vazo® 51),2,2'-azobis(2-methylpropanenitrile), 2,2'-azobis(isobutyronitrile)(Vazo® 64), 2,2'-azobis(methylbutanenitrile),2,2'-azobis(methylbutyronitrile) (Vazo® 67),1,1'-azobis(cyclohexanecarbonitrile) or 1,1'-azobis(cyanocyclohexane)(Vazo® 88)

Examples of photoinitiators include 2-butoxy-1,2-diphenylethanone,2,2-dimethoxy-1,2-diphenylethanone, a mixture ofoligo(2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl)propanone)+2-hydroxy-2-methyl-1-phenyl-1-propanone,oligo(2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl)propanone), amixture of2,4,6-trimethylbenzophenone+4-methylbenzophenone+oligo(2-hydroxy-2methyl-1-(4-(1-methylvinyl)phenyl) propanone), a mixture ofoligo(2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl)propanone)+2-hydroxy-2-methylphenyl1-propanone, 2,4,6-trimethylbenzoyldiphenylphosphineoxide+methylbenzylphenone, a mixture of2,4,6-trimethylbenzophenone+4-methylbenzophenone, a mixture of2-isopropylthioxanthone, ethyl 4-(dimethylamino)benzoate andmethyl-benzophenone, benzophenone and diaryl iodoniumhexafluoroantiomonate

Examples of miscellaneous initiators include triaryl sulfoniumhexafluorophosphate+propylene carbonate and triaryl sulfoniumhexafluoroantimonate+propylene carbonate.

To obtain stable mixtures, the initiator should be compatible with theacrylic resin and preferably, the acrylic resin is inhibited with themethyl ether of hydroquinone. Preferred curing initiators (andinhibitors) provide a-formulation which remains stable for months,preferably at least from six months to one year, preferably in excess oftwo years. Such curing initiators are typically heat activated attemperatures well above ambient temperature and more typically above 75°C. An example of a preferred free-radical initiator which can beactivated at temperatures above 75° C., including temperatures above theglass transition temperature of polymethylmethacrylate acrylic resin(Tg=105° C.), is t-butyl peroxybenzoate. Activation temperatures in therange of 75° C. to 200° C. can be used with t-butylperoxybenzoate andare often preferred.

The curing initiator can be used in amounts analogous to the amountsused in conventional acrylic-based molding compositions, which typicallyrange from about 0.12 to 1.0 weight % of the molding composition, moretypically about 0.3 to 1.0 weight %.

A critical element of the compositions of the present invention is thefiber reinforcement. These fibers are "long" fibers. The phrase "longfiber", as used herein, is intended to refer to those fibers having anaspect ratio, which is the ratio of fiber length to fiber diameter(L/D), that is theoretically large enough to result in fiber fracturenear the midpoint when stressed. Long fibers comprised of conventionalreinforcement materials have an aspect ratio exceeding 5:1. The lowestvalue for the aspect ratio at which this first occurs is referred to asthe "critical aspect ratio." The critical aspect ratio defines thecritical length at which a certain diameter fiber is considered "long".Fibers of different materials such as, for example, aramid, glass,graphite, etc., have different critical aspect ratios. In addition,identical fibers embedded in different matrices such as, for example,matrices of acrylic, epoxy, and polyester resins, have differentcritical aspect ratios. Examples of critical aspect ratios and criticallengths for various reinforcements and matrices are shown in Table 1below.

                  TABLE 1                                                         ______________________________________                                        Fiber      Matrix     (l/d).sub.c l.sub.c                                     ______________________________________                                        E-Glass    Polypropylene                                                                            140         1.78 mm                                       E-Glass Epoxy  34 0.43                                                        E-Glass Polyester 100 1.27                                                    Carbon Epoxy  47 0.33                                                         Carbon Polycarbonate 106 0.74                                               ______________________________________                                         *Engineering Materials Reference Book, 2nd Ed., p. 77, Ed. Michael            Bauccio, ASM International, 1994                                         

The aspect ratio for fibers within a matrix comprised of a cured acrylicresin will be well above 5:1 for commercially available fiberreinforcements. Typically, the aspect ratio will be above 50:1 and it isoften above 150:1. Conventional reinforcement fibers of glass, aramid,graphite, etc. having a length as low as 0.25 mm can function as longfibers within the compositions of this invention once cured since thefibers are thin and their aspect ratios are high.

While the lower limit for the preferred lengths of the long fibers isabout 0.25 mm, the long fibers can be continuous, i.e. no measurablelimit, when the molding composition is in the form of a sheet. The longfibers utilized in the molding doughs provided by this invention do havean upper limit for the preferred fiber lengths of about 6.5 mm.Preferred lengths for continuous fibers are at least 1 inch. Suitabletypes of fibers are 1) inorganic crystals or polymers, such as fibrousglass, quartz fibers, silica fibers and fibrous ceramics, which includealumina-silica (refractory ceramic fiber), boron fibers, silicon carbidewhiskers or monofilament metal oxide fibers, includingalumina-boria-silica, alumina-chromia-silica, zirconia-silica, and thelike; 2) organic polymer fibers, such as fibrous carbon, fibrousgraphite, acetates, acrylics (including acrylonitriles), aliphaticpolyamides (e.g., nylons), aromatic polyamides, polyesters, flax,polyethylenes, polyurethanes (e.g., spandex), alpha-cellulose,cellulose, regenerated cellulose (e.g., rayon), jutes, sisals, vinylchlorides, e.g., vinyon, vinyldienes (e.g., saran) and thermoplasticfibers; 3) metal fibers, such as aluminum, boron, bronze, chromium,nickel, stainless steel, titanium and their alloys; and 4) "Whiskers",which are single, inorganic crystals.

The reinforcing fibers preferably comprise such materials as glass,metals, carbon, rayon, cellulose acetate, cellulose triacetate and thelike, Mylar™ polyester, aramid/Kevlar®, Nomex™ aramid fiber orpolyethylene fiber in continuous or discontinuous form. A preferredfiber is silanized chopped glass fiber. The preferred length of fiberreinforcement utilized with the acrylic-based doughs such as bulkmolding compounds (BMC), particularly Elvacite® 2051 bulk moldingcompounds, falls in the range of 0.25 to 6.5mm. The length of fiberreinforcement utilized with vinyl ester BIS-GMA doughs preferably rangesfrom 0. 1 to 6.5mm. Fibers can be used in an amount of from 10 wt. % upto about 90 wt. % for sheet materials. In dough molding compositionssuch as BMC, levels of fiber reinforcement above 25 wt. % show littleadvantage, although higher levels such as 50 wt % can be easily used.The dough molding compositions (BMC) preferably have at least 10 wt. %long fiber. Sheet molding compounds (SMC) can use discontinuous orcontinuous reinforcing fibers, filaments, braided, knit or wovenfabrics.

A fiber reinforced composite is formed upon cure of the thermosettingmolding compositions of the present invention. Where the thermosettingmolding composition provides a composite with discontinuous fibers, thestress along the fiber is not uniform. There are portions along eachfiber end where the tensile stresses are less than that of a fiber thatis continuous in length. This region is often called the fiberineffective length. The tensile stress along the fiber length increasesto a maximum along the middle portion of the fiber. If the fiber issufficiently long (critical length) so that the ratio of the length todiameter, or aspect ratio, equals or exceeds the critical aspect ratio,the mid-fiber stress will be equal to that of a continuous filament.

The critical aspect ratio which would result in fiber fracture at itsmid-point can be expressed as (l/d)_(c) =S_(f) /2Y. Where (l/d)_(c) =thecritical aspect ratio, l=length of the fiber and w=width of the fibers,S_(f) is the tensile stress of the fiber and Y is the yield strength ofthe matrix in shear or the fiber-matrix interfacial shear strength,whichever value is lower.

If the fiber is shorter than the critical length, the stressed fiberwill de-bond from the matrix and the composite will have low strength.When the length is greater than the critical length, the stressedcomposite will not de-bond the fibers and will exhibit high strength.

The rule of mixtures for discontinuous fiber composites may be expressedas S_(c) =V_(f) ·S_(f) (l-l_(c) /2l)+V_(m) S_(m) where S_(c) is thetensile strength of the composite, S_(m) is tensile strength of thematrix, l is the actual length of the fiber, l_(c) is the criticallength of the fiber, V_(f) is the volume fraction of the fiber and V_(m)is the volume fraction of the matrix. For the composite to have a higherstrength than its matrix, a minimum V_(f) must be exceeded. This valuemay be 0.1 or greater for the plastic matrix composites. Because of highstress concentrations at the discontinuities that occur at the fiberends, tensile strength of a discontinuous fiber composite will be fromabout 55% to 86% of the fiber tensile strength and the modulus canapproach 90% to 95% of the corresponding continuous fiber composite.

The molding compositions of this invention can contain conventionaladditives where desired to obtain a particular additive effect either inprocessing or in the finished product. These include mechanical propertymodifiers, processing aids, surface property modifiers, physicalproperty modifiers, electrical property modifiers, and conventionalthickeners, such as Scott Bader's Crystig™ thickeners. Specificadditives include anti-blocking agents, anti-caking agents, anti-foamingagents, antioxidants, anti-slip agents, anti-static agents, blowingagent, coupling agents, compatibilizers, dispersing aids, flattingagents, inhibitors, catalysts, accelerators/promoters, heat stabilizers,light stabilizers, wetting agents, plasticizers, extenders,thixotropics, flame, fire and smoke retarders, internal mold releases,lubricants, impact modifiers, tougheners, coloring/dyes/pigments,odorants and deodorants, low profile or low shrink additives, lowpressure additives, clarifying agents, opacifiers, thickeners, viscositycontrol agents, permeability modifiers, biodegrading agents, flameretardants, foaming agents, blowing agents, solvents and waxes can beused. Conventional colorants can be used, such as dyes or pigments whennecessary. In dental appliances, titanium dioxide and cadmium (peachcolored) pigments are often used. The amount of colorant typicallyranges from about 0.1-1.0 wt. % of the molding composition. Othersuitable additives are dispersing agents, typically used in an amount of1 to 8 wt. % of the molding compositions. An example of suitabledispersing agent is fumed silica sold under the trade name Cab-O-Sil®.Other additives include surfactants and mold release agents. Suitablemold release agents are stearate/sterol alcohol and suitable surfactantsare di-octylsulfosuccinate (sodium salt). The mold release agents aretypically used in an amount of from 0.2-1.0 wt. % of the moldingcomposition and the surfactants are used in the amount of 0.01 to 0.5weight % of the molding composition.

Although the compositions of this invention contain fibers asreinforcement, it may still be desirable to add additional fillers,either inorganic or organic, to reduce shrinkage and distortion andimprove the physical properties of the resulting composite. Preferredexamples of inorganic fillers include silicate glass, fused silica,quartz and silanated glass ballotini. Others include calcium carbonatesof various forms and origins, silica of various forms and origins,silicates, silicon dioxides of various forms and origins, clays ofvarious forms and origins, calcium sulfates, micas, talcs, wood invarious forms, glass (milled, platelets, spheres, micro-balloons)plastics (milled, platelets, spheres, micro-balloons), recycled polymercomposite particles, metals in various forms, metallic oxides orhydroxides, metal hydrides or metal hydrates, carbon particles orgranules, alumina (tabular), aluminum powder, aramid, bronze, carbonblack, carbon fiber, cellulose, alpha cellulose, coal (powdered),cotton, fibrous glass, graphite, jute, molybdenum disulfide, nylon,orlon, rayon, silica (amorphous), sisal fibers, fluorocarbons, woodflour, kaolin, flax, zirconia and Feldspar. Although the moreconventional metal oxide fillers such as magnesium oxide and calciumoxide do not inhibit the immediate physical properties of the resultingproduct and are suitable for the molding compositions of this invention,they do cause the molding compositions to thicken over time andtherefore, molding compositions essentially free of these metal oxidesthickeners are preferred. Metal oxide thickeners form ionic polymernetworks with resin carboxyl groups requiring high pressure to breakthese bonds.

As to the organic fillers, the solid acrylic resin can function as apolymer filler when used as a thickener, but participates in thereaction unlike conventional fillers, an example being Elvacite®2051(ICI) which is a thermoplastic polymethyl methacrylate free ofbenzoyl peroxide catalytic initiator. The organic fillers which do notreact are typically used in an amount from about 0 to 30 wt. %, butcompositions of this invention with preferred levels of organic fillerstypically range from 0 to 20 wt. %, based on the total compound.

Compositions of this invention can be prepared using conventional mixingequipment such as a high shear blender. The components of the moldingcomposition are preferably first combined into two separate portions, aliquid mixture portion and solid mixture portion. The liquid mixtureincludes the liquid monomer acrylic resin, oligomer or polymer (vinylester resin, or polyester resin) optionally surfactant and catalyst. Thedry ingredients are mixed thoroughly in a high shear blender andtypically include the solid acrylic polymer as filler, colorants,dispersing agents. Preferably, the reinforcing fibers are not blendedinto the solid mixture. Following preparation of the solid and liquidmixture portions, the two portions are combined in a low shear mixer forabout five minutes, following which the reinforcing fibers are slowlyadded over an extended period. The fiber reinforcement is mixed so thatthere is no agglomeration of fibers and a uniform distribution isobtained by wetting these fibers. Once the fibers have been distributedthroughout the liquid component, the mixture is allowed to stand(mature) for about two to five days with occasional stirring. Thistechnique provides a bulk molding compound (BMC) consistent with thepresent invention. In forming sheet molding compounds (SMC), a mixtureof liquid and solid components or a single component formulation areapplied to a continuous fiber network of either knit, woven or braidedfabrics or loose-lay filaments.

With the appropriate curing initiator blended therein, the thermosettingmolding composition can be hardened by the application of heat orexposure to UV or visible light. The methods of this invention aresuitable for preparing compositions with no polymerization initiator orwith polymerization initiators that are active or inactive at ambienttemperature. The vinyl ester resin blends are well suited for use withcuring initiators that are activated by exposure to bright light. Of theheat cured resins, those which are activated at temperatures above 75°C. are preferred. Such temperatures are typically above the glasstransition temperature of the acrylic resin which forms the matrix. Thethermosetting molding compositions can be conveniently cured attemperatures of from 75° C. to 200° C. in an oil bath.

The compositions of this invention are well suited for producingdentures, inlays, crowns, bridge work, orthodontic devices, etc.However, these molding compositions are not confined to uses within thefields of dentistry (crowns, inlays), orthopedics (casts, splints) andpodiatry. These compositions can be used in industrial applications suchas model making and the production of utensils, automotive parts,bathroom fixtures and wherever enhanced physical properties must becombined with weatherability and ease of processing. The composition ofthis invention can be used in conventional matched die compressionmolding operations with conventional equipment or low compressionmolding operations, including low temperature cure techniques. Thecompositions of this invention can also be used in manual lay upapplications in the fabrication of repair of articles. The compositioncan also be used as a purging compound to scour extruder barrels ofmolders and compounders.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The following preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever.

In the foregoing and in the following examples, all temperatures are setforth in degrees Celsius; and, unless otherwise indicated, all parts andpercentages are by weight. In addition, unless otherwise indicated, allresin formulations are inhibited with trace amounts of eitherhydroquinone or methylethyl hydroquinone.

The disclosure of all patents and publications mentioned above and belowand copending application Ser. No. 08/621,723, filed Mar. 28, 1997,which will issue as U.S. Pat. No. 5,747,553 on May 5, 1998, are herebyincorporated by reference.

Protocol A

Stable, reinforced, thermoset, molding doughs of this invention wereprepared according to the following protocol:

Phase 1: combine by TOTAL weight percent and mix separately as parts Aand B:

Part A--The Liquid:

46 parts methylmethacrylate monomer;

8 parts ethylmethacrylate monomer;

4.5 parts ethyleneglycol dimethacrylate (cross-linker);

0.2 parts t-butyl peroxybenzoate (initiator);

0.1 parts dioctylsulfosuccinate, sodium salt (surfactant) and

A trace of methylethylhydroquinone (inhibitor).

Mix liquids thoroughly in a high shear blender, e.g. Lightin™ shearblender for 5 minutes.

Part B - The Powder:

1.0-8.% dispersing agent (Silica);

Colorants: TiO2, dyes and/or pigments;

33 parts Elvacite® 2051 (ICI);

5 parts Aramid PULP, no. 543; and

0.2 parts (peach) pigment.

Mix powders very thoroughly in a high shear blender for 5 minutes.

Phase 2: Combine liquid and powder by adding Part B , powder, to Part A,liquid, in a low shear mixer, e.g., Ross, Double Planetary machine, andmix for 45 minutes. Transfer to a sealed container and allow to stand(mature) for 1-4 days at 70° F. Occasional turning or stirring may beneeded during maturation, depending on the formula, to incorporate free,liquid monomer.

The end product is a thick, pliable putty with a minimal tack.

Protocol B:

Stable, reinforced, thermoset, molding doughs of the present inventionwere prepared as follows:

Phase 1: Combine by weight and mix, separately, parts A and B:

Part A--The Liquid: A terpolymer resin mixture of:

70 grams methyl methacrylate resin;

20 grams ethyl methacrylate resin;

10 grams Bis-GMA vinyl ester resin, e.g., Nupol™ Bis-GMA vinyl esterresin 046-4005;

0.25 grams catalyst (t-butyl peroxybenzoate, or t-butyl hydro-peroxide,peroxy-ketals or VAZO™ catalyst);

0.14 grams di-octylsulfosuccinate (sodium salt), a surfactant.

Portion B--The Powder: Mix very thoroughly in a high shear blender:

5.0 grams silica dispersing agent;

0.4 grams Colorants: "Cadmium" pigment;

40.0 grams methylmethacrylate polymer filler, e.g., Elvacite™ 2051methylmethacrylate polymer (ICI), which is free from benzoyl peroxidecatalytic initiator;

6.0 grams fibrillated polyethylene, Short Stuff™ polyethylene, for moldlubrication, reduction of shrinkage and distortion, and to physicallystabilize the suspension of the various powders in the mixture;

2.0 grams Calcium silicate (optional) to facilitate processing;

20.0 grams silanated, glass ballotini.

Weigh out, but do not mix in high shear blender,

25.0 grams Reinforcing fibers: glass, metals, carbon, nylon, aramidfiber (KEVLAR®), especially in its fibrillated forms, e.g., DuPont's540-543 aramid fiber (KEVLAR®) pulp, or, Nomex® aramid fiber.

Phase 2:

1) Place 75 grams of Portion A--The Liquid in a low shear mixer, addPortion B--The Powder and mix slowly and thoroughly for 5 minutes.

2) Slowly add 25.0 grams of silanated, chopped glass fiber and continueto mix for 15 minutes.

Cover the mix in a sealed container and allow to stand (mature) for 2-4days. Occasional stirring or turning may be needed, depending on theformula, to incorporate free liquid resin.

The end product is a thick, pliable putty with a minimal tack.

A typical bulk molding formulation is as follows:

    ______________________________________                                                         wt. %                                                        ______________________________________                                        solid acrylic      26                                                           liquid monomer 2.5                                                            calcium carbonate 50                                                          zinc stearate (mold release) 1                                                t-butyl peroxide benzoate .5                                                  glass fibers (1/4 in) 20                                                    ______________________________________                                    

Sheet molding compound is composed of basically four principleingredients: the thermosetting components (liquid monomer, oligomer orpolymer, and solid acrylic resin), fiber reenforcement, additives andfillers. It is feasible to use various types of specific ingredients foreach of the four principle ingredients such that an almost indefinitenumber of formulations are possible.

In the compounding process for SMC formulations, the ingredients aremixed together in two different batches, most commonly referred to asPart A and Part B. Part B includes the solid acrylic resin as its primeingredient; the additives and fillers act as carriers. Part A includesthe liquid monomer, except the glass fibers. Parts A and B are pumpedseparately at a predetermined ratio and are mixed completely to form thefinal paste immediately prior to loading the doctor box. This ratio isnormally maintained in the range 1:1 to 20:1 to obtain the bestmetering, as well as mixing of ingredients. The paste is then applied totwo carrier films to form a sandwich layer with glass fibers in themiddle.

The compounded sheets are then stored to age in a controlledenvironment. The maturation period (normally 2 to 5 days) is, in effect,the time needed for the paste viscosity to reach a level sufficient formolding. The paste viscosity at the time of compound is typically below40,000 cps (mPa·s), whereas at the time of molding, the viscosity ispreferably 20×10⁶ to 30×10⁶ cps.

    ______________________________________                                        SMC Formulation with Thermoplastic Resin                                           SMC Paste         Part by Wt.                                                                             Range                                        ______________________________________                                        Solid acrylic resin                                                                              60        15-25                                              Liquid monomer 100 20-25                                                      Thermoplastic resin 40  5-12                                                  Calcium carbonate 150 20-40                                                   (3-5 μm particle size)                                                     t-butyl peroxybenzoate 1.5                                                    Zinc stearate (mold release) 4.0                                              Fiber glass - Chopped 125 20-40                                               (1 inch)                                                                    ______________________________________                                    

EXAMPLES 1-4

Bulk molding compounds of this invention prepared in accordance withProtocol A. The components of these molding compounds are shown in theTable below:

    ______________________________________                                        Example No.      1       2       3     4                                      ______________________________________                                        INGREDIENT & Weight Percent                                                                    Unreinforced Dough Vehicles:                                 Resin, Liquid                                                                   Methyl methacrylate 61.00%  30.50%                                            Ethyl methacrylate  61.00% 30.50%                                             Other Acrylic Resins or    63.00%                                             Copolymers                                                                    Reinforcement                                                                 Mineral Filler                                                                Organic Filler                                                                Coupling Agent                                                                Internal Mold Release Agent                                                   Colorant                                                                      Curing Agent(s)                                                               t-butyl peroxybenzoate  0.30%  0.30%  0.30%  0.30%                            Thickener                                                                     Cab-O-Sil, fumed silica  2.00%  2.00%  2.00%  2.00%                           Acrylic Polymer Powder* 36.70% 36.70% 36.70% 34.70%                           Low Profile Additive:                                                         Any & All additives must be                                                   Benzoyl Peroxide (BPO) free.                                                ______________________________________                                         *All thickening and/or polymer powder is benzoyl peroxide free. Example:      ICI's, Elvacite 2051, or, Elvacite 2697.                                 

EXAMPLES 5-8

Examples 5-8 describe bulk molding compounds of this invention preparedin accordance with protocol A. The components of the molding compoundsare shown in the Table below.

    ______________________________________                                        Example No.:     5       6       7     8                                      ______________________________________                                        INGREDIENT & Weight Percent                                                     Resin, Liquid                                                                 Methyl methacryate 43.11% 35.00% 40.00%                                       Ethyl methacrylate  8.11% 8.10%                                               Other Acrylic Resins or    52.05%                                             Copolymers                                                                    Reinforcement                                                                 Glass Fiber 15.00% 15.00%                                                     Others**   15.00% 15.00%                                                      Mineral Filler                                                                Silica                                                                        Glass/Quartz 20.00% 20.00% 9.00%                                              Feldspar   5.00%                                                              Organic Filler    13.00%                                                      (Example: Powdered                                                            polyethylenes)                                                                Coupling Agent                                                                Silane 2.00% 2.00% 2.00% 2.00%                                                Internal Mold Release Agent                                                   (Example: Magnesium stearate)                                                 Colorant 0.30% 0.30% 0.30% 0.30%                                              Curing Agent(s)                                                               t-butyl peroxybenzoate 0.25% 0.25% 0.30% 0.30%                                Azo-FRS (DuPont VAZO                                                          Catalysts)                                                                    Peroxyester                                                                 Peroxy Ketal     Other Possible Catalysts                                       Thickener                                                                   Acrylic Polymer Powder                                                                         17.34%  17.34%  15.35%                                                                              15.35%                                   Silica 2.00% 2.00% 2.00% 2.00%                                                Low Profile Additive:                                                         Polyethylene Powder or Pulp   3.00%                                         ______________________________________                                         **Includes those selected from metal fibers and flakes, phosphate fiber,      Wallostonite, Dawsonite, Micro Fiber glass, processed mineral fiber, TISM     (old Fibex), magnesium oxysulfate fiber (MOS)                                 NOTE: Reinforcements and fillers are so numerous that they cannot be          specifically named.                                                      

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

What is claimed is:
 1. A method of preparing a shelf-stable moldingcomposition which comprises:a) mixing a solid acrylic resin, which isfree of active free-radical polymerization initiators, with(i) one ormore liquid monomers, liquid oligomers, liquid polymers or a combinationthereof with vinyl unsaturation, which polymerizes in the presence of anactivated free-radical polymerization initiator, wherein said solidacrylic resin absorbs said liquid monomers, liquid oligomers, liquidpolymers and combinations thereof, and reacts with said liquid monomers,liquid oligomers, liquid polymers and combinations thereof, in thepresence of an activated free-radical polymerization initiator; and (ii)long fiber reinforcement having an aspect ratio L/D greater than 5:1,which is insoluble in said solid acrylic resin; and b) aging the mixtureof solid acrylic resin, and liquid monomers, liquid oligomers, liquidpolymers or combinations thereof, and long fiber reinforcement for atleast 24 hours to allow absorption of the liquid monomer, liquidoligomer, liquid polymer or combination thereof containing vinylunsaturation, by the solid acrylic resin.
 2. A method as in claim 1,wherein said long fiber reinforcement is of a length at least about 0.25mm and the particle size of the solid acrylic resin falls within therange of 0.005 mm to 0.5 mm.
 3. The method as in claim 1 comprising theadditional step of mixing a free-radical polymerization initiator withsaid mixture of solid acrylic resin, liquid monomers, oligomers,polymers or combination thereof containing vinyl unsaturation, and longfiber reinforcement.
 4. A method as in claim 3, wherein the free-radicalpolymerization initiator is inactive at ambient temperature or itsactivity can be restrained under ambient conditions.
 5. A method as inclaim 3 comprising the additional step of heating the solid acrylicresin before mixing to inactivate any free radical polymerizationinitiators therein.
 6. A method as in claim 3, wherein the free-radicalpolymerization initiator is selected from the group consisting of ketoneperoxides, alkyl peroxides, aryl peroxides, peroxy esters, perketals,peroxydicarbonates, alkylhydroperoxides, diacyl peroxides, VAZOcompounds, photoinitiators and heat labile photoinitiators.
 7. A methodas in claim 4, wherein the free-radical polymerization initiator isactivated by exposure to UV light, visible light or a temperaturegreater than 75° C.
 8. A method as in claim 1 which comprises a solidacrylic resin selected from the group consisting of linear homopolymers,copolymers or block copolymers of acrylate or methacrylate monomers andthe liquid monomer is selected from acrylic monomers, methacrylicmonomers, styrene monomers and the liquid oligomers is selected fromacrylic oligomers, methacrylic oligomers, styrene oligomers, vinyl esteroligomers and polyester oligomers.
 9. A method as in claim 1, whereinthe liquid monomer is selected form the group consisting of acrylic acidmonomers, methacrylic acid monomers, acrylate monomers, methacrylatemonomers, vinyl ether monomers, acrylonitrile monomers, propylenemonomers, vinyl acetate monomers, vinyl alcohol monomers, vinyl chloridemonomers, vinylidine chloride monomers, butadiene monomers, isobutadienemonomers, isoprene monomers, divinyl benzene and mixtures thereof.
 10. Amethod as in claim 1, wherein said fiber reinforcement is selected fromthe group consisting of glass fibers, carbon fibers, metal fibers, rayonfibers and organic polymer fibers.
 11. A method as in claim 1, whereinthe solid acrylic resin absorbs 90% of said liquid monomers, oligomers,polymers or combination thereof, in a period of two hours or more.
 12. Amethod as in claim 1, wherein the long fiber reinforcement comprises 15wt. % to 50 wt. % of the total composition.
 13. A method as in claim 1,wherein the viscosity of the composition increases with absorption bythe solid acrylic resin of at least 90% of the liquid monomers, liquidoligomers, liquid polymers or combination thereof, and substantialviscosity build is delayed for at least two hours after the solidacrylic polymer is mixed with the liquid monomer, liquid oligomer,liquid polymer or combinations thereof.
 14. A method as in claim 13,wherein the absorption of at least 90% of the liquid monomers,oligomers, polymers or combination thereof, by the solid acrylic resinis complete in 1 to 4 days from mixing the solid acrylic polymer withthe liquid monomers, liquid oligomers, liquid polymers or combinationsthereof.
 15. A composition which comprises:a) a liquid monomer, liquidoligomer, liquid polymer or combination thereof with vinyl unsaturation,which polymerizes in the presence of an activated free-radicalpolymerization initiator; b) at least 35 wt. % based on the total weightof the liquid monomer, liquid oligomer, liquid polymer or combinationthereof in the composition, of a solid acrylic resin which is soluble insaid liquid monomer, liquid oligomer, liquid polymer or combinationthereof containing vinyl unsaturation, having an average particle sizein the range of 0.005 mm to 0.5 mm with at least a portion of saidliquid monomer, liquid oligomer, liquid polymer or combination thereofcontaining vinyl unsaturation, absorbed therein, which is free of activefree-radical polymerization initiators and which reacts with the liquidmonomer, liquid oligomer, liquid polymer or combination thereofcontaining vinyl unsaturation, absorbed therein in the presence of anactivated free-radical polymerization initiator; c) at least 10 wt. %,based on the total weight of the composition, of long fiberreinforcement having an aspect ratio (L/D) greater than 5:1 and anaverage length of at least 0.25 mm which is insoluble in the solidacrylic resin; and d) a free-radical polymerization initiator, theactivity of which can be restrained under ambient conditions or isinactive at ambient temperature so as to provide a shelf life of atleast one month at ambient temperature, wherein said composition is freeof alkali earth metal oxide fillers.
 16. A composition as in claim 15,wherein the free radical initiator is selected from the group consistingof:ketone peroxides alkyl peroxides aryl peroxides peroxy estersperketals peroxydicarbonates alkylhydroperoxides diacyl peroxides VAZOcompounds photoinitiators and heat labile photoinitiators.
 17. Athermosetting molding composition as in claim 15, wherein thefree-radical polymerization initiator is activated by exposure toultraviolet light, visible light or a temperature above 75° C.
 18. Acomposition as in claim 15, wherein the particles of solid acrylicpolymer have an average size of about 0.1 mm.
 19. A composition as inclaim 15, wherein the amount of solid acrylic resin within the moldingcomposition ranges from 35 to 70 wt. %, based on the total weight ofliquid monomer, oligomer, polymer or combination thereof in saidcomposition.
 20. A composition as in claim 15 which comprises a solidacrylic resin selected from the group consisting of linear homopolymers,copolymers or block copolymers of acrylate or methacrylate monomers andthe liquid monomer is selected from acrylic monomers, methacrylicmonomers, styrene monomers and the liquid oligomer is selected fromacrylic oligomers, methacrylic oligomers, styrene oligomers, vinyl esteroligomers and polyester oligomers.
 21. A composition as in claim 15,wherein the liquid monomer is selected from the group consisting ofacrylic acid monomers, methacrylic acid monomers, acrylate monomers,methacrylate monomers, vinyl ether monomers, acrylonitrile monomers,propylene monomers, vinyl acetate monomers, vinyl alcohol monomers,vinyl chloride monomers, vinylidine chloride monomers, butadienemonomers, isobutadiene monomers, isoprene monomers, divinyl benzene andmixtures thereof
 22. A composition as in claim 15 in the form of a bulkmolding compound having a viscosity suitable for molding at thetemperatures and pressure employed in low pressure molding equipment,wherein said long fiber reinforcement has an average length of at least1 mm and comprises from 15 to 50 wt. % of the total molding composition.23. A molding composition as in claim 15 in the form of a sheet moldingcompound having a viscosity suitable for molding at the temperatures andpressures employed in low pressure molding equipment, wherein said longfiber reinforcement has a length greater than 1 inch and comprises from25 to 70 wt. % of the total molding composition.
 24. A composition as inclaim 15, wherein said fiber reinforcement is selected from the groupconsisting of glass fibers, carbon fibers, metal fibers, rayon fibersand organic polymer fibers other than acrylic resin fibers.
 25. Acomposition as in claim 15, wherein at least 90% of said liquid monomer,liquid oligomer, liquid polymer or combination thereof, is absorbed bysaid solid acrylic resin.
 26. A composition as in claim 15, wherein thelong fiber reinforcement comprises 15 wt. % to 50 wt. % of the totalcomposition.
 27. A composition which comprises:a) a liquid monomer,liquid oligomer, liquid polymer or combination thereof containing vinylunsaturation which polymerizes in the presence of an activatedfree-radical polymerization initiator; b) 35-70 wt. %, based on thetotal weight of the liquid monomer, liquid oligomer, liquid polymer orcombination thereof in the composition, of a solid acrylic resin whichis soluble in said liquid monomer, liquid oligomer, liquid polymer orcombination thereof containing vinyl unsaturation, which has an averageparticle size in the range of 0.005 mm to 0.5 mm, which has at least aportion of said liquid monomer, liquid oligomer, liquid polymer orcombination thereof containing vinyl unsaturation absorbed therein, andwhich is free of free-radical polymerization initiators; c) at least 10wt. % based on the total weight of the composition of long fiberreinforcement having an aspect ratio (L/D) greater than 5:1 and anaverage length of at least 0.25 mm,wherein said composition is shelfstable for at least one month and is free of alkali earth metal oxidefilers and active free-radical polymerization initiators.
 28. Acomposition as in claim 27, wherein the long fiber reinforcementcomprises 15 wt. % to 50 wt. % of the total composition.
 29. Acomposition which comprises:a) a liquid monomer with vinyl unsaturationwhich cures to a thermoset resin in the presence of an activatedfree-radical polymerization initiator; b) at least 35 wt. %, based onthe total weight of the liquid monomer in the composition, of a solidacrylic resin which is essentially free of active free-radicalpolymerization initiators; c) at least 10 wt. %, based on the totalweight of the composition, long fiber reinforcement having an aspectratio (L/D) greater than 5:1; and d) a free-radical polymerizationinitiator selected from the group consisting of ketone peroxides, alkylperoxides, aryl peroxides, peroxy esters, perketals, peroxydicarbonates, alkyl hydroperoxides, diaryl peroxides, VAZO compounds,photoinitiators and heat labile photoinitiators.
 30. A composition as inclaim 29 which additionally comprises a solid resin, other than a solidacrylic resin, which polymerizes in the presence of an activatedfree-radical polymerization initiator.